A new type of touch sensor for detecting contact pressure at human fingertips is presented. Unlike traditional electronic gloves, in which sensor pads are placed between the fingers and the environment surface, this new sensor allows the fingers to directly contact the environment without obstructing the human's natural haptic senses. The finger touch force is detected by measuring changes in the coloration of the fingernail; hence the sensor is mounted on the fingernail rather than on the fingertip. Specifically, the fingernail is instrumented with miniature light emitting diodes (LEDs) and photodetectors in order to measure changes in the reflection intensity when the fingertip is pressed against a surface. The changes in intensity are then used to determine changes in the blood volume under the fingernail, a technique termed "reflectance photoplethysmography." A hemodynamic model is used to investigate the dynamics of the blood volume at two locations under the fingernail. A miniaturized prototype nail sensor is designed, built, and tested. The theoretical analysis is verified through experiment and simulation.
This paper presents combined thermal and mechanical models of a wet shape memory alloy (SMA) wire actuator. The actuator consists of a SMA wire suspended concentrically in a compliant tube. Actuation occurs as hot and cold water that are alternately pumped through the tube to contract and extend the wire, respectively. The thermomechanical model presented in this paper accounts for the nonuniform temperature change of the SMA wire due to alternating the temperature of the flow along the wire. The thermal portion of the model consists of analysis of the heat transfer between the fluid and the SMA wire. Heat loss to the environment and the temperature change of the fluid through the actuator are taken into account. Based on this analysis, the temperature of the wire at segments along its length can be determined as a function of time. The mechanical portion of the model approximates the strain-martensite fraction and martensite fraction-temperature relationships. By combining the thermal and mechanical models, the displacement of the wire can be determined as a function of time. The combined thermomechanical model will be useful for predicting the performance of wet SMA actuators in a variety of applications.
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